Regulatory Capital Requirements (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier One Capital	$ 1,158,252	$ 873,217
Capital Conservation Buffer	4.9796%
Common Equity Tier 1 Capital to Risk Weighted Assets	11.7421%	11.645%
Common Equity Tier One Capital Required for Capital Adequacy	$ 443,886	$ 337,439
Common Equity Tier 1 Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets	4.50%	4.50%
Common Equity Tier One Capital Required for Capital Adequacy plus capital conservation buffer phase-in	$ 567,187	$ 384,306
Common Equity Tier 1 Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets plus capital conservation buffer phase-in	5.75%	5.125%
Common Equity Tier One Capital Required for Capital Adequacy - minimum required plus fully phased in capital conservation buffer	$ 690,489	$ 524,906
Common Equity Tier 1 Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets plus capital conservation buffer fully phased-in	7.00%	7.00%
Total Capital, Actual Amount	$ 1,280,326	$ 947,436
Total Capital (to risk-weighted assets), Ratio	12.9796%	12.6347%
Total Capital For Capital Adequacy Purposes, Amount	$ 789,130	$ 599,892
Total Capital (to risk-weighted assets), For Capital Adequacy Purposes, Ratio	8.00%	8.00%
Capital Required for Capital Adequacy plus capital conservation buffer phase-in	$ 912,432	$ 646,759
Capital Required for Capital Adequacy to Risk Weighted Assets plus capital conservation buffer phase-in	9.25%	8.625%
Capital Required for Capital Adequacy plus capital conservation buffer fully phased-in	$ 1,035,734	$ 787,359
Capital Required for Capital Adequacy to Risk Weighted Assets plus capital conservation buffer fully phased-in	10.50%	10.50%
Tier 1 Capital, Actual Amount	$ 1,165,903	$ 874,688
Tier 1 Capital (to risk-weighted assets), Ratio	11.8196%	11.6646%
Tier 1 Capital For Capital Adequacy Purposes, Amount	$ 591,848	$ 449,919
Tier 1 Capital (to risk-weighted assets) For Capital Adequacy Purposes, Ratio	6.00%	6.00%
Tier One Risk Based Capital Required for Capital Adequacy plus capital conservation buffer phase-in	$ 715,149	$ 496,786
Tier One Based Capital Required for Capital Adequacy to Risk Weighted Assets plus capital conservation buffer phase-in	7.25%	6.625%
Tier One Risk Based Capital Required for Capital Adequacy plus capital conservation buffer fully phased-in	$ 838,451	$ 637,386
Tier One Based Capital Required for Capital Adequacy to Risk Weighted Assets plus capital conservation buffer fully phased-in	8.50%	8.50%
Tier 1 Capital, Actual Amount	$ 1,165,903	$ 874,688
Tier 1 Capital (to average assets), Ratio	10.9611%	9.5526%
Tier 1 Capital For Capital Adequacy Purposes, Amount	$ 425,469	$ 366,263
Tier 1 Capital (to average assets) For Capital Adequacy Purposes, Ratio	4.00%	4.00%
Banking Subsidiaries [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier One Capital	$ 1,184,476	$ 862,381
Capital Conservation Buffer	4.8123%
Common Equity Tier 1 Capital to Risk Weighted Assets	12.0133%	11.5051%
Common Equity Tier One Capital Required for Capital Adequacy	$ 443,687	$ 337,304
Common Equity Tier 1 Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets	4.50%	4.50%
Common Equity Tier One Capital Required for Capital Adequacy plus capital conservation buffer phase-in	$ 566,933	$ 384,152
Common Equity Tier 1 Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets plus capital conservation buffer phase-in	5.75%	5.125%
Common Equity Tier One Capital Required for Capital Adequacy - minimum required plus fully phased in capital conservation buffer	$ 690,180	$ 524,696
Common Equity Tier 1 Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets plus capital conservation buffer fully phased-in	7.00%	7.00%
Common Equity Tier One Risk Based Capital Required to be Well Capitalized	$ 640,881	$ 487,217
Common Equity Tier 1 Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	6.50%	6.50%
Total Capital, Actual Amount	$ 1,263,252	$ 935,129
Total Capital (to risk-weighted assets), Ratio	12.8123%	12.4756%
Total Capital For Capital Adequacy Purposes, Amount	$ 788,777	$ 599,652
Total Capital (to risk-weighted assets), For Capital Adequacy Purposes, Ratio	8.00%	8.00%
Capital Required for Capital Adequacy plus capital conservation buffer phase-in	$ 912,023	$ 646,500
Capital Required for Capital Adequacy to Risk Weighted Assets plus capital conservation buffer phase-in	9.25%	8.625%
Capital Required for Capital Adequacy plus capital conservation buffer fully phased-in	$ 1,035,269	$ 787,043
Capital Required for Capital Adequacy to Risk Weighted Assets plus capital conservation buffer fully phased-in	10.50%	10.50%
Total Capital To Be Well Capitalized Under Prompt Corrective Action Provision, Amount	$ 985,971	$ 749,565
Total Capital (to risk-weighted assets) To Be Well Capitalized Under Prompt Corrective Action Provision	10.00%	10.00%
Tier 1 Capital, Actual Amount	$ 1,184,476	$ 862,381
Tier 1 Capital (to risk-weighted assets), Ratio	12.0133%	11.5051%
Tier 1 Capital For Capital Adequacy Purposes, Amount	$ 591,582	$ 449,739
Tier 1 Capital (to risk-weighted assets) For Capital Adequacy Purposes, Ratio	6.00%	6.00%
Tier One Risk Based Capital Required for Capital Adequacy plus capital conservation buffer phase-in	$ 714,829	$ 496,587
Tier One Based Capital Required for Capital Adequacy to Risk Weighted Assets plus capital conservation buffer phase-in	7.25%	6.625%
Tier One Risk Based Capital Required for Capital Adequacy plus capital conservation buffer fully phased-in	$ 838,075	$ 637,130
Tier One Based Capital Required for Capital Adequacy to Risk Weighted Assets plus capital conservation buffer fully phased-in	8.50%	8.50%
Tier 1 Capital To Be Well Capitalized Under Prompt Corrective Action Provision, Amount	$ 788,777	$ 599,652
Tier 1 Capital (to risk-weighted assets) To Be Well Capitalized Under Prompt Corrective Action Provision, Ratio	8.00%	8.00%
Tier 1 Capital, Actual Amount	$ 1,184,476	$ 862,381
Tier 1 Capital (to average assets), Ratio	10.8186%	9.4275%
Tier 1 Capital For Capital Adequacy Purposes, Amount	$ 437,939	$ 365,902
Tier 1 Capital (to average assets) For Capital Adequacy Purposes, Ratio	4.00%	4.00%
Tier 1 Capital To Be Well Capitalized Under Prompt Corrective Provision, Amount	$ 547,423	$ 457,378
Tier 1 Capital (to average assets) To Be Well Capitalized Under Prompt Corrective Provision, Ratio	5.00%	5.00%